This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Colours are among the most important aspects of a film. For good viewing, it is essential that a film reproduced for example in a cinema is as close as possible to the film director's intentions. It is thus necessary to characterise the digital reproduction device, e.g. the projector, to find out its colour behaviour, as this enables the modification of the input signals so that the displayed colours correspond as much as possible to the intended colours.
Another way of putting this is that the colour output values of reproduction devices are device dependent. An exemplary device dependent colour space is a Red Green Blue (RGB) colour space. This means that the same input colour values fed into two different reproduction devices may yield different output colours, because the colour space rendering associated with the respective devices are not identical.
As a consequence, to keep the same output colour it is often necessary to transform the colours of a specific image from one colour space to another when the reproduction device is changed. The transformation of colour values from one colour space to another is usually non-linear.
A necessary prerequisite for colour value transformation is to quantify, i.e. to measure colour values. The physiological colour impression of an observer is determined by the spectrum of the light entering into the observer's eye and the subsequent visual processing of the human brain. The human eye has three types of receptors for colour vision. Thus, it is possible under constrained viewing conditions to define a specific colour by a set of three values, which are called CIE tristimuli XYZ. The tristimuli have already been defined in 1931 by the CIE (Commission Internationale de l'Eclairage).
The tristimuli for a specific colour are calculated from the spectrum S(λ) of the colour by integration with weighting functions:X=∫S(λ)x(λ)dλ  (1)Y=∫S(λ)y(λ)dλ  (2)Z=∫S(λ)z(λ)dλ  (3)
The tristimuli XYZ are still objective values even if weighted by weighting functions related to the human eye. Numerous measuring instruments such as photometers or video-photometers use XYZ tristimuli to give objective measures of colours.
In practice, the transformation of colour values from one device dependent colour space to another device dependent colour space includes an intermediate step. The intermediate step is to transform a device dependent colour space first into a device independent colour space. The intermediate step is a specific colour transformation called device model. A device model is established by colour characterization.
Colour characterization consists of the establishment of a colour transformation between device dependent colour values and device independent colour values. This transformation is usually calculated from measurements of device dependent and device independent colour values. For a reproduction device this procedure is as follows: A set of device dependent colour values are fed into the device and the reproduced colours are measured by an objective, optical measurement instrument giving a set of corresponding device independent colour values, for example measured in the XYZ or L*a*b* colour spaces.
One problem of device characterization is the high number of measurements necessary to establish a colour transform with acceptable precision. Since for practical reasons the number of measurements is limited, a problem becomes how to choose the limited number of measurements or how to increase the possible number of measurements within a given time.
When for example measuring a display device, a limited number of RGB input colours to be measured have to be selected. In an EPFL research report from 2002 of D. Alleysson and S. Susstrunk entitled “Caractérisation couleur dans la chaine cinématographique numérique: Projection et visualisation” (in English: Colour Characterization of the digital cinema chain), the authors propose to choose values for (R,G,B) of the type (n,0,0), (0,n,0), (0,0,n), (n,n,0), (n,0,n) and (0,n,n) with values for n between 0 and 255 in steps of 15 or 30.
A further problem is that in certain environments, light levels are not within the standard range of the color acquisition devices. In cinemas, for instance, as an example the maximum luminance may be of the order of 48 Cd/m2 while the luminance associated with the darkest colours are in the order or less than 0.1 Cd/m2.
Using colour acquisition devices in a standard manner (i.e. performing a direct measure of the displayed image) leads to noisy measures in the dark levels with quite long acquisition times, which is aggravated if, as above, this has to be repeated for many colour values.
It can therefore be appreciated that there is a need for a solution that obtains accurate measurements, particularly at low light levels, and decreases the measurement time as compared with standard methods.
The present invention provides such a solution.